1. Field of the Invention
The invention relates to a chill block melt spinning apparatus for producing flake-type powder of the smallest magnitude.
2. Description of Related Art
Continuous casting processes for producing metal wire or filaments from molten metal are well known in the art. The earliest practices of continuous casting of metals are found in the manufacture of shot in shot towers in the early 1800's. Continuous casting processes evolved into three primary categories: (1) processes wherein molten metal is contained on all sides; (2) processes wherein there is always a free surface, exemplified by chill block melt spinning; and (3) processes wherein molten metal is formed without a mold, such as by levitated casting or the continuous casting of hollow metal microspheres.
Chill block melt spinning produces long thin ribbons or extremely short flakes. The technique consists of ejecting molten metallic material through a small orifice to produce a continuous jet of liquid. If the jet remains molten for long times or over long distances, the surface tension forces of the material cause the jet to be broken up into droplets. If solidification of the jet occurs before this breakup then a round metallic wire is produced. If solidification occurs after the disintegration then shot or spherical powders are produced. The molten jet can be acted upon by causing it to impinge on a rotating chill block which may be a wheel or a curvilinear block. The surface velocity of the chill block at the point where the molten jet impinges is referred to as the "impingement velocity" (V.sub.i). "Ejection velocity" (V.sub.e) is the velocity of the molten stream just prior to contacting the chill surface. If the impingement velocity is approximately equal to the ejection velocity then a continuous ribbon having a width slightly larger than the jet diameter is produced. If V.sub.i is large relative to V.sub.e then a continuous, thin ribbon will be formed. The width of this ribbon will still be slightly larger than the jet diameter. As V.sub.i continues to increase relative to V.sub.e, the ribbon becomes thinner until the jet can no longer supply enough material to maintain a continuous ribbon and the product becomes staple ribbon fiber. The length of the staple fiber becomes less as V.sub.i is further increased until finally a flake powder is produced.
There are six variables which can be manipulated to change the product produced by this type of process. These are orifice (jet) diameter, temperature of the molten alloy, surface tension of the alloy, ejection velocity, impingement velocity and the attack angle at which the jet approaches the chill block.
In applicant's own prior system, four rotating chill cylinders are located symmetrically around a rotating cup to provide a contact surface for streams or droplets travelling radially outward from the cup to form the droplets into flakes, to cool the metal and deflect the flakes. Such a system is shown with reference to FIGS. 1 and 2. In FIG. 1, there is shown a portion of a chill block melt spinning apparatus 10 including four rotating chill cylinders 12 which are individually driven by motors 14 through shafts 42. The cylinders 12 are dispersed in a square around a rotating cup 16. In operation, with reference to FIG. 2, molten alloy 18 is fed from a supply 20 to rotating cup 16 which is driven by a motor 30. The alloy 18 is centrifugally ejected radially from the cup 16 in sheets 22 extending along a horizontal plane. As the alloy 18 travels outward, it is disintegrated into droplets 24 by surface tension before contacting rotating chill cylinders 12. This allows each droplet 24 to be attenuated into a flake 26 before solidifying. The flakes 26, after contacting an outer surface 28 of the cylinders 12, are deflected downward into a collection area 46. The collection area 46 may include a collection box 48. The collection box 48 may also include a micro mesh filter 50 and a vacuum pump 52. A drawback to this system is uneven size distribution. Another drawback is the likely occurrence of droplets to travel between the cylinders, causing accumulations on drive motors and shaft members which will increase down time for repair and maintenance.
U.S. Pat. No. 2,825,108 discloses a method and apparatus for making metallic filaments. The apparatus includes a base having a rotating chill block which may include a spherical or ellipsoidal cavity. Molten metal is directed through an orifice onto the rotating chill block. The size of the filament produced is dependent on the ejection velocity of the molten metal out of the orifice and the impingement velocity of a surface of the chill block at which the molten metal impinges. By changing the ejection velocity equal to or less than that of the impingement velocity, small discontinuous flakes can occur.
U.S. Pat. No. 4,474,604 discloses a method of producing metal powder. The powder is formed by heating and melting a metal in a vacuum chamber. Molten metal in the form of droplets falls from the chamber and collides with a surface of a roll rotating at a high speed so as to pulverize and partially solidify the droplets. Particles formed by the roll are scattered and collected in a collecting box. An alternative embodiment uses a radially contracted mid portion on a roll to aid in conveying the flow of particles from the roll and to reduce particle size.
U.S. Pat. No. 3,797,978 discloses an apparatus for disintegrating a stream of ferroalloy into solidified spheroidal configurations. A melted stream of ferroalloy is subjected within an insulating zone to a centrifugal force and is radially dispersed into contact with a curvilinear surface. The curvilinear surface is shown as being part of a rotatable drum which rotates about a vertical axis. The apparatus also includes a shell which prevents stray material from exiting the apparatus.
U.S Pat. Nos. 4,468,241 and 4,808,097 disclose methods and apparatus for forming fibers from meltable materials. Meltable material is supplied to a distribution bowl which radially flings the melt over a rim of the bowl. The melt, as it is radially travelling outward from the bowl, encounters a jet stream of gas which breaks up forced downward by the stream and gravity into a collector housing.
U.S. Pat. No. 4,435,342 discloses a method of producing ultra fine particles by delivering a molten stream of metal onto a rotating primary annular surface, discharging molten fine droplets from an edge of the annular surface against an inclined second annular surface. The droplets upon contacting the second annular surface are subdivided and discharged to be cooled as ultra fine particles.
U.S. Pat. No. 4,613,076 discloses an apparatus and method for forming fine liquid metal droplets. The apparatus comprises a rotatable member situated in a pressurized or evacuated chamber. An electric field is provided near an edge of the rotatable member to overcome surface tension of the metal. Molten metal is directed onto a surface of the rotating member and as the liquid is thrown from the edge of the rotating member, the electric field causes fine metal droplets to form.
The need exists for an apparatus which can produce a metallic alloy flake or particle which is of the smallest magnitude. Although numerous devices have been devised to produce small metal particles, none have produced consistent flake dimensions approaching 1 micron. An additional problem with prior art devices is the inability to produce a consistently narrow distribution of particle sizes. The need exists for a method or apparatus which does not build up layers of solidified droplets on chill surfaces, thus providing an apparatus which is capable of producing a consistent flake yield and distribution throughout a production run.